1. Field of the Invention
The present invention relates to a semiconductor memory device which includes a circuit including a semiconductor element such as a transistor.
2. Description of the Related Art
Semiconductor memory devices are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile semiconductor memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in the semiconductor memory device is selected and charge is stored in a capacitor connected to the transistor.
In a DRAM, since charge of a capacitor is lost instantaneously after information is read out, rewriting needs to be performed every time information is read out. In addition, when a transistor included in the DRAM is off, charge is lost due to leakage current (off-state current) between a source and a drain and the like; therefore, a data retention time is short. Accordingly, a writing operation (a refresh operation) is needed in a predetermined cycle, which results in increase in power consumption. In addition, data is lost when supply of power is stopped, and thus another memory device using, for example, a magnetic material or an optical material is needed in order to retain data for a long time.
Another example of a volatile semiconductor memory device is a static random access memory (SRAM). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, there is a problem in that cost per storage capacity becomes high because a circuit such as a flip flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile memory device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by retaining charge in the floating gate. Therefore, a flash memory has advantages in that the data retention time is extremely long and refresh operation which is necessary in a volatile memory device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in the semiconductor memory device deteriorates by tunneling current generated in writing, so that the semiconductor memory device stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for the semiconductor memory devices is equalized is employed, for example. However, a complicated peripheral circuit is needed to achieve this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for a semiconductor memory device in which data is frequently rewritten.
In addition, high voltage is necessary to retain charge in the floating gate or remove the charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to retain or remove charge, so that it is not easy to increase the speed of write and erase operations.